Chromizing and analogous methods



Patented Jan. 7, 1958 CHROMIZING AND ANALOGOUS METHODS Philippe Galmiche, Paris, France, assignor to Otlice National dEtudes et de Recherchcs Aeronautiques 0. N. g; R. A., Chatillon-sous-Bagneux, France, a society of ance No Drawing. Application January 18, 1955 I I I 7 Serial No. 482,675 Claims priority, application France June 27, 1950 4 Claims. (Cl. 204-33) The present invention relates to methods of forming on metal pieces metallic protective layers one of which is constituted by a chromium alloy. Its chief object is to provide a method of this kind which is better adapted to meet the requirements of 1 jecting said piece at high temperature to the action of o fluoride vapors at least partly constituted by chromium fluoride while keeping said piece out of contact with any halide other than a fluoride in the vapor state.

It is'known'that the protection of metal pieces against chemical agents is commonly obtained by forming, on the surface thereof, a relatively thin layer of an addition metal or alloy.

There are known processes which consist in superimposing upon the metal which constitutes the piece a film of an addition metal inthe pure state, which can be achieved, for instance,'through electrolytic processes, by projection in the pulverulent state (shooping) or by immersion. 1 Now, when a ferrous piece is subjected to direct electrolytic chromium plating, this piece is but imperfectly protected against corrosion due to the fact that there are risks of oxidizing of the foundation, constituted by the ferrous metallic surface, on which chromium is being deposited during the electrolytic treatment, such risks resulting from the porosity of electrolytically deposited metal layers and in particular chromium layers. Inorder to attenuate this defect and to improve adhesion, it is usual to proceed to preliminary scouring and buffing of the foundation, which in many cases must receive a first copper deposit and a second nickel deposit, on which the chromium plating is effected.

This method is relatively complicated and expensive.

Other known methods consist in forming, on the surface of the pieces to be protected, an intermetallic dif fusion alloy having the required protective qualities, such an alloy being for instance obtained by calorizing (case of the addition metal consisting of aluminium) chromizing, simple or mixed cementation, and so on.

When the addition metal is to consist chiefly of chromium, it has been endeavoured, in order to obtain stronger and thicker protective layers, to form mixed diffusion alloys, but the process is delicate in view of the different rates of diffusion of the addition metals in the metal of the pieces to be treated and of the different conditions in which the respective addition metals achieve satisfactory diffusion.

I A 'known method consists also in first electrodepositing a chromium layer on the piece to be protected and then heating the piece surrounded by powdered chromium with the purpose of improving the fixation of superficial chromium on the piece. But the electrolytic layer of chromium obtained after the first step will peel oif at places during the subsequent heating treatment (expansion coeflicients of iron and chromium very different from each other) and contact of the piece with powdered chromium will give a very defective surface.

The object of my invention is to provide a method of forming on a metal piece at least two superficial protective layers one of which is constituted by a chromium alloy, this method eliminating the drawbacks inherent in the known processes above referred to.

According to my invention, I subject said piece to two different successive treatments each for forming a protective metallic layer on the surface of the piece, one of these layers being constituted by a chromium alloy, the treatment for the formation of said chromium alloy protective layer being performed before any superficial addition of chromium to said piece and consisting in subjecting said piece at a sufiiciently high temperature to promote the chromizing action of fluoride vapors at least partly constituted by chromium fluoride while keeping said piece out of contact with any halide other than a fluoride in the vapor state as described in my copending U. S. application, 'Ser. No. 572,455, filed March 19, 1956, which is a continuation-in-part of Ser. No. 175,502, filed July 24, 1950, and now abandoned.

The treatment by chromium fluoride vapors may take place either before or after the other treatment. However, since, as above specified, the chromium fluoride vapor treatment must, according to my invention, be performed before any superficial addition of chromium to the piece, the other treatment, if it consists of a chromium electroplating, must necessarily be performed after the chromium fluoride vapor treatment.

I will first consider the case where the treatment by means of chromium fluoride vapors (also called chromi- Zing treatment) is performed before the other treatment.

In particular, such a chromizing treatment will be performed before the other treatment when said second treatment is to form a protective layer of a metal (other than chromium) having a tendency to diffuse too rapidly in the metal of the piece, which is the case of aluminium added by calorizing on ferrous pieces.

For instance, it was observed that ferrous pieces coated with aluminium resist very well to corrosion at high temperature, in particular in the case of an atmosphere containing sulfide derivatives. Unfortunately, the aluminium introduced by calorizing difiuses very rapidly in iron (the cotflicient of diffusion of aluminium at 1100" C. in D.l0 flux per sq. cm. and per diem is as compared with 5.9 for chromium in the same conditions) and forms a thick and brittle layer.

A preliminary chromizing treatment of the pieces, by means of chrominum fluoride vapors, makes it possible to obtain a chromium deposit which acts as a screen when aluminium is subsequently diffused in the piece at high temperature.

By way of example, mild steel pieces chromized for six hours at 1100, and then calorized for ten hours at 950/900 in a cementing mixture containing 30% of aluminium, 65% of alumina and 5% of ammonium chloride, resist to dry corrosion for several hundreds of hours at 1100/1125 in combustion gases containing substantial amounts of sulfurous anhydride. Similar results are obtained by metalizing of the piece after chromizing by projection of aluminium with a thickness of 0.5 mm.

Preliminary chromizing also gives satisfactory results in the case of a protection by silicon or molybdenum.

Another example where the treatment by fluoride vapors is performed before the other treatment is the method which comprises first forming by means of said fluoride vapors, on the piece to be treated, a superficial diffusion alloy of the metal of the piece with chromium and then electroplating with a metal which is preferably also chromium.

Advantageously, at the beginning of this second treatment (electrodepositing of a metal) the current is reversed for a short time so as to improve the conditions of adhesion of the electrolytic deposit.

Pieces obtained according to this particular embodiment of my invention have the same external aspect as if they had undergone a long electroplating treatment and these pieces resist remarkably well to corrosion, owing to the fact that the chromium layer adheres perfectly well to the chromium alloy foundation, which is itself proof against oxidizing.

By way of indication, it may be considered that the preliminary thermo-chemical treatment to which the pieces are subjected makes it possible to reduce by about 75-90 percent the duration of the electrodepositing treatment. This greatly reduces the cost and the drawbacks (lack of uniformity of the deposit) inherent in electroplating processes.

As for the preliminary thermo-chemical treatment for obtaining a diffusion chromium alloy layer on the surface of the piece to be treated, it is advantageously carried out, as indicated in the aforementioned copending U. S. application or the French Patents No. 576,473, of August 5, 1949, Addition No. 47,655, of December 6, 1949, and Addition No. 48,355, of June 27, 1950, by subjecting the metal piece to be treated exclusively to the action of chromium fluoride vapors. Advantageously, a chromium reserve is placed in this reaction chamber, above or in contact with the piece to be treated.

Preferably, a certain amount .of hydrogen is maintained in the atmosphere of the reaction chamber.

The temperature and duration of treatment vary according to the desired depth of penetration of the superficial alloy to be obtained. For instance, for treating a mild steel piece containing 0.1% of carbon, a treatment of four hours at 1100 C. gives a depth of penetration of 0.2 mm.

I will now consider the case where the treatment by fluoride vapors is performed after the other treatment. In this case, as above specified, said other treatment must form a protective layer of a metal or metals other than chromium.

Such a method makes it possible to form protective layers of chromium alloy on pieces of a metal which practically forms no solid solution with chromium, such a metal being for instance copper or a copper alloy.

It is well known that a very adhesive layer of chromium would constitute an excellent protective coating on copper pieces (or copper alloy pieces), in particular pieces exposed to combustion gases at high temperature and in particular gases containing lead tetraethyl).

It is also known that it is always of interest, in order to obtain the best possible fixation of the protective metal layer, to perform a solid diffusion of the protective metal into the metal of the piece.

Now it happens that chromium can practically give no solid solutions into copper or copper alloys (the percentage of chromium being at most 0.2%).

In order to obviate this difiiculty, according to my invention, I first form on the piece a layer of an intermediate metal, for instance nickel, capable of forming a solid solution both with chromium and with copper or copper alloys. Then, by means of a thermal treatment making use of fluoride vapors, I apply a layer of chromium which forms by solid interdiffusion an alloy with the intermediate nickel layer.

I thus obtain a very good fixation, by solid interdiffu- 4 sion, of the intermediate nickel layer both with the copper piece and with the protective layer of chromium applied over said nickel layer.

The nickel layer may be applied electrolytically, its binding with the copper piece being then achieved either by a special thermal treatment, or by the thermal treatment which serves to form the chromium protective layer over the nickel layer.

1 therefore finally obtain metal layers which adhere perfectly to one another and the composition of which varies, starting from the inside of the piece, from copper (or a copper alloy) to cupro-nickel, then nickel and finally nickel-chromium.

Concerning the electrolytic treatment for depositing the intermediate nickel layer, it will be advantageous to use nickel plating baths giving good deposits.

By way of example, I will give two bath compositions which have proved to be advantageous for practical purposes:

Gr. per liter Crystallized nickel sulfate 200 Sodium chloride 1S Anhydrous sodium sulfate 50 with a pH approximating 6 or 6.5, a temperature of the bath averaging 30 C. and a current density ranging from 1 to 3 amp/sq. dm.

with a pH averaging 6, at ordinary temperature (from 15 to 20 C.) and a current density ranging from 0.5 to l amp/sq. dm.

With such baths, the duration of the treatment may be of some hours.

The chromizing treatment which follows this electroplating is performed at temperatures which do not exceed 900 or 1000 0., according to the composition of the pieces, in order to avoid risks of melting, since copper melts at 1063 C. and aluminium bronze melts at temperatures ranging from 925 to 1050 C.

This chromizing treatment may advantageously last for some hours, for instance about ten hours at a temperature ranging from 950 to 975 C.

The fluoride vapors will be advantageously formed in situ and the pieces will be embedded in a mass of regeneration chromium, as set forth in my parallel patent applications.

On the other hand, it is advantageous, before performing the chromizing treatment, to subject the intermediate metal layer to a degassing which may be performed for instance by heating at 150 C. approximately.

By applying this treatment I obtain chromium-nickel layers which are perfectly adherent to the copper of the piece and the thickness of which may reach 0.1 mm. with an average percentage of chromium of 12. Spectral analysis combined with electrolytic polishing made it possible to show the progressive variation of the cromium percentage and of the nickel percentage from the surface, the chromium percentage decreasing gradually whereas the nickel percentage first increases, passes through a maximum, then decreases. The pieces have a satisfactory superficial appearance and a perfect resistance to dry oxidizing in ethyl containing combustion gases up to temperatures of about 800 C.

A final chromizing treatment performed after the other treatment (or treatments) makes it possible to achieve a very satisfactory bonding between two or more metallic layers deposited on the piece to be protected provided that the layers thu's applied are sufficiently ductile (Ni, Co, etc.). As a matter of fact, an intermediate diflusion of the layers already applied takes place during the chromizing process.

On the other hand, it is possible, by the combined use of at least two treatments, to introduce into the final protective layer one or several metals the application of which through an electrolytic process would be diflicult or would produce irregular layers (case of aluminium, molybdenum, tungsten).

It then becomes possible from an industrial point of view to apply a succession of metals which would be incompatible with the conventional electrolytic methods, for instance cobalt and molybdenum simultaneously, by electrolysis, then chromium by diffusion from chromium fluoride vapors, or cobalt and tungsten simultaneously, by electrolysis, then chromium by diffusion from chromium fluoride vapors. Whereas, by electrolysis, it is impossible to obtain a satisfactory depositing of chromium on layers of Co and M0 or Co and W, chromium superficial addition can be obtained on such layers with very good results by difiusion at high temperature from chromium fluoride vapors.

With my invention it is also possible to superimpose, by chromium fluoride vapor treatment, a chromium layer on a cobalt layer, or a chromium layer on a manganese layer. I

This is due to the fact that these electrolytically formed layers are adhesive, little porous and, being sufliciently ductile, will not peel off during the high temperature treatment with chromium fluoride vapors.

Of course the metal initially added must not have a melting point lower than the temperature at which it diffuses in one of the two metals in contact therewith.

For instance, this condition is fulfilled by nickel, cobalt, manganese with respect to chromium and ferrous pieces.

The advantages of using fluoride vapors of the addition metal (chromium) for the thermo-chemical diffusion treatment are extremely important and satisfactory results could not be obtained with vapors of another halide, for the following reasons:

(a) In the case of a chromizing treatment after which an electroplating treatment (advantageously with chromium) is performed, the initial treatment by means of chromium fluoride vapors gives a layer which is smoother and has a higher protective value and which can therefore easily undergo the slight polishing which is generally performed after electroplating. Furthermore, as the chromium diffusion layers thus obtained are much thicker, for the same conditions of temperature and duration of the treatment, rectification or polishing of the angular portions of the piece becomes possible as there are very little risks of destroying the layer.

(b) In the case of protection against dry corrosion by means of mixed chromium and aluminium layers deposited on ferrous pieces, the results obtained when the first treatment, that is to say chromizing, is carried out by means of a fluoride of chromium, are much better. The protective layer which includes iron, chromium and aluminium, has the maximum of efficiency when the respective proportions are approximately: 40% for iron, 35% for chromium and for aluminium. In this case, it is quite unnecessary, in order to obtain a very good protection, to oxidize, by means of a special treatment, the superficial layer in order especially to form a protective layer of alumina.

With the method according to my invention, I obtain after a treatment of 6 hours at a temperature of 1100 C. a ferrous piece having a chromized layer of a thickness of 0.5 mm. and in which the richness in chromium is 60%.

After a calorizing treatment which may be carried out for about ten hours at a temperature of approximately 950 C., the relative proportions are substantially those above stated.

These optimum proportions are obtained inside a substantial layer because chromium is present up to that 'of 0.6 to 0.7 mm.

When a thick and rich layer of chromium is formed on the surface of the piece, aluminium as deposited by this" calorizing treatment is efiiciently prevented from penetrating too quickly into the mass of the piece and if the temperature is suitably adjusted, aluminium does not go beyond the chromized layer, whereby there is not formed an underlying layer of lower mechanical resistance. 7

The excellent results that are obtained are due to the important ratio of the diffusion coeflicients of aluminium into iron and to that of chromium into iron (ratio of 170 to 5.8 according to Bardenhauer and Miiller-W. Seith, Diffusion in Metallen, Berlin). Thus, aluminium can diffuse into the protective layer without the percentage of chromium therein decreasing too much and too quickly as a function of the depth.

(0) The fluorides of the metals that are used for preliminary electrolytic depositings are all much less volatile than other halides (see the table given hereinafter). Therefore the risk of the initial depositing being partly volatilized, which exists chiefly in the angular portions of the piece, is reduced to a minimum. This volatilizing tends to take place chiefly at the beginning of the treatment when the pieces the surface of which is not yet saturated with chromium are in contact at high temperature with halogen derivatives.

In some cases (such as the above described protection of copper or copper alloy pieces by diffusion of chromium into an intermediate nickel layer), local volatilizing of the intermediate deposit unavoidable with halides other than chromium fluoride would fully prevent the addition of chromium (since copper and chromium are not compatible with each other).

Here is a table indicating the boiling points of the respective halides.

Boiling points of the respective halides Fluorine, Chlorine, Bromine, Iodine, degrees degrees degrees degrees 1. 304 1.049 decomposes 1. 973 i 1. 040 182 200 360 1. 758 713 963 713 1. 050 732 650 624 (C 13 Fe) 1. 000 315 sublimates. sublimates (ver 250 It will be noted that fluorides have boiling points much higher than those of the other halides.

Owing to these properties of fluorides, it is possible, after nickel plating or cobalt plating of an iron piece, to add chromuim by diffusion at 1075 C. without forming volatile fluorides of cobalt or nickel, whereas, at the same temperature, if chromium addition was carried out by means of a chloride, cobalt or nickel would be eliminated in the form of a chloride.

It is necessary, in order to obtain these advantages, to take care that the piece to be treated is in contact with chromium fluoride only in the vapor phase.

This application is a continuation-in-part of my application Ser. No. 233,500, filed on June 25, 1951, and now abandoned.

What I claim is:

l. The method of protecting the surface of a base metal piece having the principal component selected from the group consisting of iron and copper comprising forming a first protective addition metal layer on the surface of said piece, said first metal being selected from the group consisting of nickel, cobalt and manganese. and subsequently exposing said piece to the action of chromium fluoride vapors at a chromizing temperature and below the melting point of said piece while preventing contact of said piece with any halide other than 9. References Cited in the file of this patent fluoride in the vapor state. UNITED STATES PATENTS 2. A method as claimed in claim 1 wherein the principalcomponent of said base metal piece is copper, and 2555372 Ramage June 1951 said first metal is nickel. 5 FOREIGN PATENTS 3. A method as claimed in claim 2 wherein said piece 321 914 Great Britain Nov 11 1929 is exposed to the action of chromium fluoride vapors 7222797 Great 1955 at a temperature range from about 950 to 975 C.

4. A method as claimed in claim 3 wherein said nickel is electroplated on the surface of said base metal piece. 10

UNITED STATES PATENT GFFHIZ? HER FKCATE ilfiiiiififlfififi Patent No. 2,819,208 January '7 1958 Philippe Galmiche It is hereby certified. that error appears in the above numbered patent requiring correction and that the said Letters Patent should. read as cor-- rected below,

In the grant, line 3,, address of assignee, for "of Chatillon-sous- Bagneus, France" read. of Ghatillonsou.s==]3agneux, Francein the printed specification, column 2, line 52, for 'cotfficient1;. read' coefficient column '6, line 13, for "of 170 to 598" read. of 1'70 to 5.9 3 line 35, for "points" read W point line '73, strike out "exposing said piece" and insert instead coating said piece by exposure Signed and sealed this 8th day of April. 1958:.

(SEAL) Attest:

KARL HQ MINE ROBERT c. WATSON Atteeting Officer Commissioner of Patents 

1. THE METHOD OF PROTECTING THE SURFACE OF A BASE METAL PIECE HAVING THE PRINCIPAL COMPONENT SELECTED FROM THE GROUP CONSISTING OF IRON AND COPPER COMPRISING FORMING A FIRST PROTECTIVE ADDITION METAL LAYER ON THE SURFACE OF SAID PIECE, SAID FIRST METAL BEING SELECTED FROM THE GROUP CONSISTING OF NICKEL, COBALT AND MANGANESE, AND SUBSEQUENTLY EXPOSING SAID PIECE TO THE ACTION OF CHROMIUM FLUORIDE VAPORS AT A CHROMIZING TEMPERATURE AND BELOW THE MELTING POINT OF SAID PIECE WHILE PREVENTING CONTACT OF SAID PIECE WITH ANY HALIDE OTHER THAN A FLUORIDE IN THE VAPOR STATE. 